This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-080093, filed Mar. 22, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a pattern formation material, pattern formation method, and exposure mask fabrication method and, more particularly, to a pattern formation material for electron beam lithography, a pattern formation method using the pattern formation material, and an exposure mask fabrication method using the pattern formation material.
Recently, chemically amplified resists are often used in the fabrication processes of semiconductor devices. These chemically amplified resists are roughly classified into positive resists and negative resists. Known examples of the positive chemically amplified resists are a ternary composition containing an alkali-soluble resin, dissolution inhibitor, and photoacid generator, and a binary composition containing an alkali-soluble resin to which a substituent (dissolution inhibiting group) having dissolution inhibiting capacity is introduced, and a photoacid generator.
When a positive chemically amplified resist is used, the dissolution of the alkali-soluble resin is inhibited in an unexposed state by the dissolution inhibitor or the dissolution inhibiting group. When this resist is irradiated with chemical radiation to generate an acid from the photoacid generator and the resultant resist is subjected to post-exposure baking (PEB), the dissolution inhibitor or the dissolution inhibiting group is decomposed by the generated acid to lose its dissolution inhibiting capacity. Since this makes an exposed portion of the positive chemically amplified resist soluble in alkali, the resist is selectively removed by development using an alkali developer.
In this chemically amplified resist, the acid generated when the photoacid generator is irradiated with chemical radiation functions as a catalyst. When this chemically amplified resist is used, therefore, a plurality of molecules can be reacted by one photon. That is, higher sensitivity than those of conventional resists can be obtained by the chemically amplified resist.
In a patterning process using the chemically amplified resist described above, a resist film is irradiated at once with ultraviolet radiation through a desired pattern, and a resist pattern obtained by developing the irradiated resist film is used as an etching mask to pattern a thin film. This resist pattern is required to decrease the pattern size and increase the shape accuracy. In effect, many proposals have been made to meet these requirements.
For example, Jpn. Pat. Appln. KOKAI Publication No. 8-262721 disclosed a binary chemically amplified resist composition containing, as alkali-soluble resins, polyhydroxystyrene in which 10 to 60 mol % of a hydroxyl group are substituted by a tert-butoxycarbonyloxy group (t-BOC group), and polyhydroxystyrene in which 10 to 60 mol % of a hydroxyl group are substituted by a residual group indicated by formula xe2x80x94OCR1R2OR3. Jpn. Pat. Appln. KOKAI Publication No. 8-262721 describes that this resist composition realizes high sensitivity, high resolution, and high heat resistance because the solubility of the alkali-soluble resins and the dissolution stopping power are well balanced by using the t-BOC group and the residual group indicated by formula xe2x80x94OCR1R2OR3 (R1 represents a hydrogen atom or methyl group, R2 represents a methyl group or ethyl group, and R3 represents a lower alkyl group) as dissolution inhibiting groups.
Jpn. Pat. Appln. KOKAI Publication No. 9-6002 describes that when the conventional chemically amplified resists are used, bridging occurs in the upper portion of an opening in the resist pattern because an acid generated by exposure is deactivated. This deactivation of the acid is brought about by ammonia or the like contained in the atmosphere in a clean room. As a solution to this problem, Jpn. Pat. Appln. KOKAI Publication No. 9-6002 disclosed a positive chemically amplified resist composition formed by further adding an organic carboxylic acid compound to the composition disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-262721. Note that Jpn. Pat. Appln. KOKAI Publication Nos. 9-6003, 9-22117, 9-127698, and 10-10738 also disclosed chemically amplified resist compositions similar to those disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 8-262721 and 9-6002.
Like Jpn. Pat. Appln. KOKAI Publication No. 9-6002, Jpn. Pat. Appln. KOKAI Publication No. 9-219355 describes that when the conventional chemically amplified resists are used, an acid generated by exposure is deactivated by ammonia or the like contained in the atmosphere in a clean room, and the section of the resist pattern assumes a T-top profile. As a method of solving this problem, Jpn. Pat. Appln. KOKAI Publication No. 9-219355 describes that the resist composition is so controlled that the film reduction amount is increased when the sectional shape of the resist pattern has a T-top profile, and that the film reduction amount is decreased when the sectional shape of the resist pattern has a sloping shoulders-like profile.
Any of the above Published Unexamined Patent Applications mainly describes that ultraviolet radiation is used to expose the chemically amplified resist disclosed in it. Recently, however, electron beam writing is beginning to be used in addition to ultraviolet exposure described above.
Theoretically, electron beam writing can increase the resolution to the beam diameter of an electron beam. That is, electron beam writing is more suitable for finer fabrication than ultraviolet exposure. Therefore, electron beam writing is used when ultrafine fabrication is necessary, such as in the fabrication of pattern formation masks, e.g., a general photomask, electron beam exposure mask, X-ray lithography mask, EUV (Extreme UltraViolet) lithography mask, and stencil mask.
Unfortunately, when an electron beam is used as chemical radiation, peculiar problems that cannot occur in ultraviolet exposure take place. For example, Jpn. Pat. Appln. KOKAI Publication No. 11-271965 describes that no resist patterns having good sectional shapes can be obtained when electron beam exposure is performed by using a chemically amplified resist containing a polymer having an acetal-type acid leaving group. Jpn. Pat. Appln. KOKAI Publication No. 11-27165 also describes that the cause of this defect is that the release reaction from a polymer having an acetal-type acid leaving group requires water, but the conventional methods cannot allow a resist film to absorb sufficient moisture. This lacking of water occurs because electron beam writing is performed in a vacuum, unlike ultraviolet exposure. As a method of solving this problem, Jpn. Pat. Appln. KOKAI Publication No. 11-271965 disclosed a method by which a resist film is allowed to absorb moisture as it is left to stand in the air between exposure and PEB.
The use of an electron beam as chemical radiation also poses other characteristic problems. When an electron beam is used as chemical radiation, exposure cannot be performed at once unlike when ultraviolet radiation is used. That is, writing must be gradually performed when an electron beam is used. Hence, in the fabrication of a pattern formation mask described above, writing of a resist film sometimes takes a long time of 10 hr or more. When this is the case, the diffused state of an acid generated by exposure of a chemically amplified resist differs in the writing end position largely from that in the writing start position.
At present, the dimensional accuracy required for electron beam writing used in the semiconductor device fabrication processes is on the nanometer order. However, some conventional technologies produce a dimensional difference of about 30 nm between the writing start and end positions. Such a dimensional difference obviously does not satisfy the above requirement and hence is a large cause of a lowering of the yield.
As described above, the influence the time required for electron beam writing has on the dimensional accuracy is extremely large. However, the known chemically amplified resists are formed assuming principally ultraviolet exposure and hence do not take these problems special to electron beam writing into consideration. In fact, the above problems are not at all described not only in the Published Unexamined Patent Publications enumerated in connection with ultraviolet exposure but also in the Published Unexamined Patent Publications enumerated in relation to electron beam writing. That is, the problems described above in connection with electron beam writing have not been solved yet.
It is an object of the present invention to provide a pattern formation material, pattern formation method, and exposure mask fabrication method capable of fabricating a pattern formation mask with high yield.
It is another object of the present invention to realize high dimensional accuracy during patterning using electron beam lithography.
According to the first aspect of the present invention, there is provided a pattern formation material for electron beam lithography, comprising an alkali-soluble resin and a photoacid generator which generates an acid when irradiated with an electron beam, wherein the pattern formation material further comprises first and second dissolution inhibiting groups each of which has a capacity of inhibiting dissolution of the alkali-soluble resin in an alkali solution and loses the capacity upon application of an acid, the first dissolution inhibiting group increasing the sensitivity of the pattern formation material when left to stand in a vacuum after irradiated with an electron beam, and the second dissolution inhibiting group decreasing the sensitivity of the pattern formation material when left to stand in a vacuum after irradiated with an electron beam, and the ratio of the first dissolution inhibiting group to the second dissolution inhibiting group is adjusted such that the size of an alkali-soluble portion, which is a portion made soluble in the alkali solution when the pattern formation material is irradiated with an electron beam, is substantially held constant independently of the standing time in a vacuum.
According to the second aspect of the present invention, there is provided a pattern formation method using a pattern formation material for electron beam lithography, the pattern formation material comprising an alkali-soluble resin and a photoacid generator which generates an acid when irradiated with an electron beam, the pattern formation material further comprising first and second dissolution inhibiting groups each of which has a capacity of inhibiting dissolution of the alkali-soluble resin in an alkali solution and loses the capacity upon application of an acid, the first dissolution inhibiting group increasing the sensitivity of the pattern formation material when left to stand in a vacuum after irradiated with an electron beam, and the second dissolution inhibiting group decreasing the sensitivity of the pattern formation material when left to stand in a vacuum after irradiated with an electron beam, comprising the steps of determining the ratio of the first dissolution inhibiting group to the second dissolution inhibiting group such that the size of an alkali-soluble portion, which is a portion made soluble in the alkali solution when the pattern formation material is irradiated with an electron beam, is substantially held constant independently of the standing time in a vacuum, forming a photosensitive film by coating the surface of an object to be processed with the pattern formation material containing the first and second dissolution inhibiting groups at the determined ratio, performing electron beam writing on the photosensitive film, and forming a patterned film by developing the photosensitive film subjected to the electron beam writing.
According to the third aspect of the present invention, there is provided an exposure mask fabrication method using a pattern formation material for electron beam lithography, the pattern formation material comprising an alkali-soluble resin and a photoacid generator which generates an acid when irradiated with an electron beam, the pattern formation material further comprising first and second dissolution inhibiting groups each of which has a capacity of inhibiting dissolution of the alkali-soluble resin in an alkali solution and loses the capacity upon application of an acid, the first dissolution inhibiting group increasing the sensitivity of the pattern formation material when left to stand in a vacuum after irradiated with an electron beam, and the second dissolution inhibiting group decreasing the sensitivity of the pattern formation material when left to stand in a vacuum after irradiated with an electron beam, comprising the steps of determining the ratio of the first dissolution inhibiting group to the second dissolution inhibiting group such that the size of an alkali-soluble portion, which is a portion made soluble in the alkali solution when the pattern formation material is irradiated with an electron beam, is substantially held constant independently of the standing time in a vacuum, forming a light-shielding film on one principal surface of a transparent substrate, forming a photosensitive film by coating the surface of the light-shielding film with the pattern formation material containing the first and second dissolution inhibiting groups at the determined ratio, performing electron beam writing on the photosensitive film, forming a patterned film by developing the photosensitive film subjected to the electron beam writing, and etching the light-shielding film by using the patterned film as an etching mask.
The term xe2x80x9cincreasing the sensitivityxe2x80x9d means that assuming the electron beam irradiation amount and the development conditions are constant, the aperture size of a pattern formed by exposing and developing a thin film made of the pattern formation material increases, i.e., means that a desired aperture size is realized with a smaller electron beam irradiation amount. On the other hand, the term xe2x80x9cdecreasing the sensitivityxe2x80x9d means that assuming the electron beam irradiation amount and the development conditions are constant, the aperture size of a pattern formed by exposing and developing a thin film made of the pattern formation material decreases, i.e., means that the realization of a desired aperture size requires a larger electron beam irradiation amount. Furthermore, the term xe2x80x9calkali-soluble portionxe2x80x9d means a portion dissolvable in an alkali solution when development is performed under predetermined conditions.
As described above, when a general chemically amplified resist is left to stand in a vacuum after electron beam writing, an acid diffuses to increase the sensitivity of the material. When a chemically amplified resist containing an acetal-type dissolution inhibiting group is used, on the other hand, it is possible to obtain a resist pattern having a good sectional shape by allowing a resist film to well absorb moisture between electron beam writing and PEB. That is, in the absence of water, no release reaction of the acetal-type dissolution inhibiting group occurs.
In addition to these facts, the present inventors noted that electron beam writing is performed in a vacuum unlike ultraviolet exposure, and examined sensitivity changes produced when the resist was left to stand in a vacuum after electron beam writing. Consequently, the present inventors have found that a chemically sensitized resist having an acetal-type dissolution inhibiting group decreases its sensitivity when left to stand in a vacuum after electron beam writing, and that when such a chemically amplified resist which decreases its sensitivity and a chemically amplified resist which increases its sensitivity are mixed at an appropriate ratio, a resist pattern having a uniform size can be formed independently of the standing time in a vacuum after electron beam writing. That is, the present invention can realize high dimensional accuracy when patterning is performed using electron beam writing. Therefore, the present invention makes it possible to fabricate a pattern formation mask and the like with high yield.
In the present invention as described above, the ratio (commonly the molar ratio) of the first dissolution inhibiting group to the second dissolution inhibiting group must be so set that the size of an alkali-soluble portion made soluble in an alkali solution by an electron beam irradiation is maintained substantially constant independently of the standing time in a vacuum, i.e., that the size variation with the vacuum standing time is substantially zero. However, this size variation need not be completely zero. That is, the size of the alkali-soluble portion can slightly change with the standing time in a vacuum.
Usually, the dimensional accuracy required for electron beam writing for fabricating an exposure mask for use in the fabrication process of semiconductor devices is xc2x110 nm. Also, the longest time required for electron beam writing for fabricating an exposure mask is normally 10 hr. Accordingly, the ratio of the first dissolution inhibiting group to the second dissolution inhibiting group need only be so controlled that the size variation is 5 nm or less for a vacuum standing time of 10 hr, when other size variation factors are taken into account. The ratio is preferably so controlled that the size variation is 3 nm or less for a vacuum standing time of 10 hr.
In the present invention, the ratio of the alkali-soluble resin, dissolution inhibiting groups, photoacid generator, and the like in the pattern formation material is determined in accordance with the necessary sensitivity, resolution, and the like, as in the case of general positive chemically amplified resists. In other words, the ratio of the alkali-soluble resin, dissolution inhibiting groups, photoacid generator, and the like has a very large influence on the essential resist characteristics such as the sensitivity and resolution of the pattern formation material.
As described above, the present invention uses the method of controlling the ratio of the first dissolution inhibiting group to the second dissolution inhibiting group. Therefore, the size variation with the vacuum standing time can be decreased to substantially zero without changing the ratio of the dissolution inhibiting groups to the alkali-soluble resin, photoacid generator, or the like. That is, the size variation with the vacuum standing time can be decreased to substantially zero with almost no effect on the sensitivity, resolution, and the like of the pattern formation material. Accordingly, the present invention eliminates the need for an enormous number of tests in determining the composition of a pattern formation material by which the above-mentioned characteristics are given.
Examples of the alkali-soluble resin usable in the present invention are polymer materials generally used in chemically amplified resists. Specific examples are polyhydroxystyrene, a hydroxystyrene-styrene copolymer, an isopropenylphenol-styrene copolymer, a hydroxystyrene-methyl methacrylate copolymer, an isopropenylphenol-methyl methacrylate copolymer, a phenol novolak resin, a cresol novolak resin, a xylenol novolak resin, poly(4-carboxyadamantane)methacrylate, and a copolymer of 4-carboxynorbornene and maleic anhydride. These polymer materials can be used singly or in the form of a mixture.
In the present invention, as the first dissolution inhibiting group which increases the sensitivity when the pattern formation material is left to stand in a vacuum after exposure, functional groups used in common chemically amplified resists can be used. Examples are a t-butoxycarbonyloxy group, t-butylester group, methoxyethoxyester group, t-butoxycarbonyloxymethoxy group, t-pentylester group, t-hexylester group, 1-cyclohexenylester group, 2-cyclopropyl-2-propylester group, 2-phenyl-2-propyl group, and 1-methoxyethylester group. The pattern formation material of the present invention can contain either only one type of a group or a plurality of types of groups selected from these functional groups, as the first dissolution inhibiting group.
In the present invention, as the second dissolution inhibiting group which decreases the sensitivity when the pattern formation material is left to stand in a vacuum after exposure, it is possible to use, for example, an acetal-type functional group represented by the following formula, tetrahydropyranyl group, silylether group, enolether compound, O,N-acetal compound, acidoacetal group, silylester compound, and orthoester group. The pattern formation material of the present invention can contain either only one type of a group or a plurality of types of groups selected from these functional groups, as the second dissolution inhibiting group. 
(wherein R1 represents a hydrogen atom or methyl group, R2 represents a methyl group or ethyl group, and R3 represents a lower alkyl group.)
The pattern formation material of the present invention can contain the above first and second dissolution inhibiting groups as functional groups which modify the alkali-soluble resin mentioned earlier. Also, the pattern formation material of the present invention can contain the above first and second dissolution inhibiting groups as a dissolution inhibitor having these dissolution inhibiting groups. That is, the pattern formation material of the present invention can be either a binary composition or a ternary composition. Furthermore, the pattern formation material of the present invention can contain one of the first and second dissolution inhibiting groups as a functional group which partially modifies the alkali-soluble resin, and the other of the first and second dissolution inhibiting groups as a functional group forming a dissolution inhibitor.
In the present invention, examples of the photoacid generator for generating an acid when irradiated with an electron beam are photoacid generators generally used in chemically amplified resists. Specific examples are onium salts and an iminosulfonate compound such as triphenylsulfonium, trifluoromethanesulfonate, and triphenylsulfodi (4-t-butylphenyl)iodoniumcamphorsulfonate, a disulfone compound, and a halogen compound. These photoacid generators can be used singly or in the form of a mixture.
The pattern formation material of the present invention can further contain a solvent and the like. Furthermore, in addition to the above components, the pattern formation material of the present invention can contain slight amounts of additives.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.